The present invention relates broadly to a demodulator apparatus, and in particular to QPSK demodulator apparatus with a two-step quadrupler and/or time-multiplexing quadrupling.
In the prior art the use of demodulators is quite well known and highly specialized. The specialized field of demodulators includes some sophisticated apparatus such as the modulation wipe off demodulator and the reverse demodulator.
The present QPSK modulation wipeoff demodulator has several serious fundamental problems relating to signal acquisition and to break-lock performance. The reverse demodulator which is a variation of the modulation wipeoff demodulator shares similar common problems. Some of these problems are that a wipeoff demodulator is incapable of distinguishing between a locked signal, an unlocked signal, or amplified receiver noise. This characteristic greatly complicates signal acquisition strategy, and in some applications makes reliable signal acquisition virtually impossible.
The wipeoff demodulator also has a large experimental difference in break-lock SNR performance between the cases of: (a) no modulation (-17 db), one channel PRN code (-4.5 db), and (c) both channels with different PRN codes (+4 db).
The reverse demodulator appears to be no better than wipeoff demodulator with respect to break lock, and to lock indication. The present invention state-of-the-art QPSK demodulator devices suffer from serious spurious signal problems which inhibit meaningful performance investigation. Spurious signals in a device of this type are a particular problem by virtue of the fact that the QPSK demodulator is basically a cross-correlator technique.
A step in the right direction is the recent developments in the field of the QPSK quadrupling (.times.4) demodulators. However, the improvements which have been achieved, have created additional problems areas. For example, the high data rates require higher receiver IF frequencies in order to handle the date bandwidths, but this causes the quadrupled output frequency to be at an unreasonably high frequency. A very wideband system may employ a 10 GHz IF but this causes the .times.4 signal to be at 40 GHz. Furthermore, it is difficult to build wideband .times.4 elements at these frequencies, and the output .times.4 frequency itself is hard to handle. The most common type of .times.4 element employs a nonlinear reactance which is subject to parametric breakup, a problem with this type of element. An improved QPSK .times.4 demodulator apparatus is provided with improved characteristics to overcome the above described prior art performance problems.